Quoted:
I would sent it to ADCO and have them cut it.
They will also check if the gas port is of right size.
They won't let you down and it's very affordable.
If you have the cash then just buy a new upper and build another lower later on for the 14.5"
That worked for me....Adco cut mine to 10.3".
Then, of course, AIM Surplus had a sale on spike's 10.5" complete uppers, and the 10.3" became a dedicated .22LR upper. The 10.5" is set up as a host for my Gemtech G5.
10.3":
10.5":
Bushmaster complete lower (Bushmaster)
Spike's Tactical 10.5" 1-7 barrel, Smith Vortex G5 mount (Aim Surplus, mount/suppressor obtained from and mounted by Gemtech)
LMT Fixed BUIS (Bravo Company)
Aimpoint M2 in Wilcox Mount (Aimpoint-SKD Tactical, Mount-Brownells)
KAC M4 RAS (EE)
Surefire Scout with KX2C Head Upgrade, Gear Sector Offset Scout Mount (EE)
Magpul B.A.D. Lever (Brownells)
Ergo Grip (Brownells)
Gear Sector Single Point Sling (Brownells)